Methylcyclopentadiene (MCP) is a key raw material in the synthesis of methylcyclopentadienyl manganese tricarbonyl (MMT). The first step of the reaction sequence involves reacting excess MCP with sodium metal in an ether solvent to generate a process intermediate complex, MCP-Na. For economic reasons, the excess MCP is recovered in the product distillation step, blended with fresh MCP, and finally recycled back to the initial reaction step.
The ether solvent used in this first step in a MMT synthesis process is a dimethyl carbitol (DMC) solvent. This solvent, commonly known as “diglyme” or diethylene glycol dimethylether belongs to a class of solvents capable of sufficiently solubilizing alkali metals, metal compounds, and their salts. One way to make “glymes” is by a controlled polymerization of ethylene epoxide with sodium methoxide and then capping the desired chain product with methyl halide. This solvent is thought to be the source of the alcohol contaminants in the MMT crude that end up distilling over with the unreacted MCP. At the elevated reaction temperature stages of generating the MCP-Na the sodium is thought to cleave a small amount of the DMC solvent to give the alkoxides of methanol and 2-methoxyethanol. These alcohols are reconstituted further down the MMT synthesis steps at the stage where the MMT crude is washed with an aqueous solution to remove the salts before feeding this crude to the distillation columns. This is how the water is introduced into the MMT crude, and the water together with the alcohols end up distilling over with the MCP stream.
In the MMT synthesis process, the water- and alcohol-laden MCP stream is recycled back and reacted with sodium metal in the DMC solvent to give the MCP nucleophile, MCP-Na. At the same time the sodium will react with the alcohol contaminants methanol and 2-methoxyethanol to give the corresponding sodium alkoxides, and with the contaminant water to give sodium hydroxide. These alkoxides and hydroxide will compete with MCP-Na chemistry further down the process for making MMT, hence compromising overall MMT yield. Finally they will be reconstituted back to their original methanol, 2-methoxyethanol, and water form at the aqueous wash of the MMT crude before being sent to the columns. These contaminants then recycle back into the MMT synthesis process as part of the MCP recycle stream. The kind of cycle these contaminants are exhibiting in this MMT process can be termed as a catalytic cycle of destroying the reactant sodium metal. If allowed to continue, this can become expensive because the levels of these contaminants in the process are being boosted as more are generated from DMC cleavage each cycle.